In the past, copper has been used as electrical conducting material for an electric wire used for a wire harness of a vehicle, for a terminal of a motor or circuit wiring used for electrical appliances or the like. The vehicle and the electrical appliances have recently been required for weight saving, the copper (its specific gravity of 8.9) used therefor has been exchanged to aluminum (Al) (its specific gravity of 2.7), the specific gravity of which is approximately one-third of that of Cu.
As a method of soldering such Al electric wire, Al terminal, Al wiring or the like (hereinafter, referred to as “Al member” expedientially), a fusion soldering method such as a dip method or a flow method is exemplified. In the dip method, the soldering is performed while a tip of Al electric wire or Al terminal is dipped into a stationary solder tank. In the flow method, the soldering is performed by contacting jet solder in a jet solder tank to Al wiring. In these soldering methods, the fusion solder in the stationary solder tank or the jet solder tank exposes the air for a long time.
By the way, until now, Sn—Zn solder alloy has been used as the solder alloy for soldering the Al member. This is because Zn has a small electrode potential difference between it and Al so that it can suppress galvanic corrosion. As the Sn—Zn solder alloy, for example, Sn-9Zn, Sn-15Zn and Sn-20Zn, which are stipulated in Japanese Industry Standard (JIS) Z 3281 and exhibit low melting points from the viewpoint of workability, are exemplified.
Furthermore, Patent Document 1 discloses Sn—Zn series solder alloy containing Ti and Al as a solder alloy which directly can solder any hard-to-soldering metal having an oxide film made of Al or the like.